1. Field of the Invention
The present invention relates to a liquid developer, method of manufacturing the liquid developer, and image forming apparatus and method, such as an electrophotographic method, using the liquid developer.
2. Discussion of the Background
An electrophotographic toner image formed on a photosensitive body is preferably transferred to a receptor, such as a transfer roller, with high transfer efficiency. There are several transfer methods for transferring the toner image from the photosensitive body to the receptor using an electric field, heat, or pressure.
In the transfer method using the electric field, the transfer roller disposed in proximity or in contact with the photosensitive body at a transfer station is discharged by a corona discharger disposed inside of the transfer roller, whereby the transfer roller has an electric charge opposite to an electrical potential of toner particles on the photosensitive body and the transfer of toner images is accommodated. The transfer method using the electric field tends to be affected by an electrical resistance of the receptor and by process conditions, such as temperature and humidity.
In another transfer method applying a direct current bias between the photosensitive body and the receptor, electrons may be injected to the toner particles, whereby the toner image texture and transfer efficiency would be deteriorated. Transfer methods using the electric field or the direct current bias have such problems whether the processes are dry toner development processes or a liquid toner development processes.
The transfer method using heat or pressure achieves high efficiency and high texture transfer without being affected by temperature or humidity However it requires some devices, such as a release coating layer of high release characteristic formed on the photosensitive body.
The toner particles of liquid developer are usually expected to fix on a receptor at low temperature without using a fixing unit; therefore a glass transition temperature Tg of resin which forms the toner particles is preferably below room temperature. However, such resin particles of low glass transition temperature Tg have higher adhesion force.
The release characteristic of a release coating layer is deteriorated through consecutive usage and the resin particles of high adhesion force tends to remain on the deteriorated release coating layer, whereby a poor transfer was observed. The resin of low glass transition temperature Tg also tends to be softer and resin particles deform to have a film shape under an applied pressure. The film shape forming toner particles may tend to stay on the photosensitive body, whereby occurrence of poor transfer may increase.
Toner particles including resin of a glass transition temperature Tg higher than room temperature is disclosed in Japanese Patent Publication (Tokukou) 63-33141. The resin forming the toner particles is dissolved in a solvent, such as a chlorine-based solvent, and mixed with pigment. The pigment mixed resin is dispersed in a carrier solvent in which the resin cannot be dissolved and the dispersed particles comprising mixed resin and pigment are adjusted to form toner particles.
The thus formed toner particles tend to have smaller adhesion force than the toner particles having the glass transition temperature Tg of lower than room temperature and also tend to retain their shape without changing to film-like shape. However, toner particles at a transfer station, where the photosensitive body and the receptor are pressed each other, receive high pressure during the transfer using heat and pressure. The resin particles forced to change its shape by the pressure tend to adhere to each other and form a toner film, whereby the image adheres to the photosensitive body and the transfer efficiency may decrease.
An object of the present invention is to provide a liquid developer, a method of manufacturing the liquid developer, and an image forming method using the liquid developer in which a high-resolution image can be obtained.
Another object of the present invention is to provide a liquid developer, and a method of manufacturing the same, whereby fine resolution and high electrical chargeability can be obtained. Although the liquid developer of the present invention is applicable to all transfer methods which are known in the art, the developer can maintain an initial characteristic through repetitious image forming process, such that the liquid developer of the present invention is specifically appropriate for an apparatus using at least a pressure or a heat transfer method.
In a first aspect of the present invention, there is provided a liquid developer including an electrically insulating solvent and a plurality of toner particles. Each of the plurality of toner particles includes resin particle and pigment particles, the resin particle being non-soluble to the electrically insulating solvent and the pigment particles being selectively formed on a surface of the resin particle.
In a second aspect of the present invention, there is provided a liquid developer including an electrically insulating solvent and a plurality of toner particles, each including a resin particle and pigment particles, the resin particle being non-soluble to the electrically insulating solvent, and the pigment particles being formed on a surface of the pigment particle, wherein a coverage rate of the surface of the resin particle by the pigment particles is 3.5% or more.
In a third aspect of the present invention, there is provided a liquid developer including an electrically insulating solvent and a plurality of toner particles, each including a resin particle and pigment particles, wherein the resin particle is non-soluble to the electric insulation solvent. Each of the toner particles includes a surface portion and an inside portion, and a first density of the pigment particles per unit volume of the resin particle at the surface portion is larger than a second density of the pigment particles per unit volume of the resin particle at the inside portion.
In a fourth aspect of the present invention, there is provided a method of manufacturing a liquid developer including preparing an electrically insulating solvent, adding a plurality of resin particles which are insoluble in the solvent and a plurality of pigment particles to the electrically insulating solvent, and milling the electrically insulating solvent with the plurality of resin particles and the plurality of pigment particles at a temperature not more than a glass transition temperature of the resin particles.
In a fifth aspect of the present invention, there is provided an image forming apparatus including a latent image retaining body, a developing unit, and an intermediate transfer medium. The developing unit is disposed adjacent to the latent image retaining body and configured to develop a latent image formed on the latent image retaining body using a liquid developer. The liquid developer includes an electric insulation solvent and a plurality of toner particles. Each of the plurality of toner particles contains a resin particle non-soluble in the electrically insulating solvent and pigment particles. Each of the toner particles includes a surface portion and an inside portion, and a first density of the pigment particles per unit volume of the resin particle at the surface portion is larger than a second density of the pigment particles per unit volume of the resin particle at the inside portion. The intermediate transfer body contacts the latent image retaining body at a transfer station and receives a pressure of about 0.5 kg/cm2 to 50 kg/cm2 from the latent image retaining body at the transfer station. The intermediate transfer body has a surface speed from about 80% to about 99% or from about 101% to about 120% of the surface speed of the latent image retaining body.